The present invention relates to a method and apparatus used during manufacture of a paper product such as paper or cardboard in order to influence the surface characteristics of the paper product.
Thus, the present invention relates to a method and apparatus to be used in connection with a paper machine for the purposes of improving the surface smoothness and gloss of paper or cardboard, the apparatus including one or more smoothing means which operate in such a way as to have a rubbing action with respect to the surface of the web which is to be smoothed.
It is of course well known that the surface smoothness and gloss of a paper product may be influenced in a paper machine by the calender treatment subsequent to the drying of the web. If such calender treatment is not provided the surface of the web will remain uneven and rough to a greater or lesser extent. This unevenness results from the manner in which the paper product is manufactured. During formation of the web at the wet end of the paper machine the forming web is contacted at one or both sides by wire and felt fabrics. These fabrics produce in the web surface a so-called marking or pattern which is determined by the structure of the wire and felt belts or fabrics. Furthermore, the unevenness and roughness of the paper will also result from shrinkage and crumpling of the web during the drying phase.
The calender treatment may be carried out with a calender assembly which forms part of the machine itself, this calender assembly being situated in the paper machine between the drying cylinder section and the reeling section. Also it is possible to provide calender treatment with a separate supercalender assembly which forms part of a structure for treating the paper subsequent to the manufacture thereof. The smoothing and burnishing action achieved both by a calender forming part of the machine or by a supercalender results from compression and deformation acting on the paper web at nips formed between rolls of such calender assemblies. However, there is no differential between the surface speed of the calender rolls and the travelling speed of the web in the nips therebetween. In other words there is no slippage between the web and the rolls. This action achieved from the machine calender as well as from a supercalender results not only in an increased smoothness and gloss of the paper product but also in a considerable reduction in the thickness of the paper and in an equalization of thickness variations, precisely as a consequence of the pressure exerted on the web in the nips between the rolls of the calenders.
With respect to the present state of the art, there is a known friction-type of calender which at one time was widely used for burnishing cardboard on one side thereof as well as for providing certain special papers of so-called high gloss. Such a friction calender includes two rolls, one of which is an elastic paper or filled roll while the other is a glossy chilled-iron roll having a smooth hard surface. The latter roll is rotated at a peripheral velocity which is 2-4 times the rotary speed of the paper roll. The paper web or cardboard web travels through the nip defined between such rolls, and at this nip the chilled roll which has a surface speed higher than the travelling speed of the web produces a burnishing action on the surface of the web which engages this chilled roll. In the event that it is desired to burnish such a web on both sides it must be directed at least twice through a pair of calender rolls. It is, however, also possible to utilize two calenders, one after the other, with one of the calenders providing burnishing at the top side of the web while the other of the calenders provide burnishing at the underside of the web.
With respect to the state of the art, reference may also be made to German Pat. Nos. 527,130 and 597,449.
Calender treatment is particularly necessary in the case of paper to be used for writing or printing. Where paper is to serve as writing paper of where it is to serve primarily for text which is to be printed, a machine calender treatment may suffice. However, if the paper is to be used for printing in connection with accurate reproduction of illustrations or for multicolor printing, then the paper is required to possess a high degree of surface smoothness and in many cases also a gloss in addition to the high surface smoothness. In such cases a paper smoothing and burnishing treatment by way of a separate supercalender is required.